When acquiring an X-ray image, an object to be examined, e.g. a patient, is arranged between an X-ray generating device and an X-ray detector. X-ray radiation emanating from the X-ray generating device is penetrating the object to be examined, subsequently arriving at the X-ray detector. The object to be examined, situated in the path of the X-ray radiation is spatially attenuating the X-ray beam, depending on the specific tissue structure within the object. The X-ray detector is subsequently detecting the spatially attenuated X-ray radiation by determining an intensity distribution of the X-ray radiation, which image information is employed for generating, further processing, and subsequently displaying an X-ray image of the object to be examined.
However, an object to be examined may provide only minor differences when attenuating the X-ray radiation, resulting in a relatively uniformly attenuated X-ray image having low contrast, thus lacking detail of the imaged inner structure of the object.
While certain objects or regions within an object may comprise similar attenuation properties, a phase of X-ray radiation penetrating the object may be influenced to a larger extent by the structure of the object.
In phase-contrast imaging, at least partly coherent X-ray radiation is employed, e.g., generated by a source grating arranged adjacent to, in the vicinity of an X-ray source, e.g. an X-ray tube. Coherent X-rays penetrating the object may allow for subsequent retrieval of phase information.
However, a phase of a wave may not be measured directly, rather a phase-shift may be required to be converted to an intensity modulation, e.g., by interfering two or more waves. For generating an according interference pattern, a so-called phase grating is employed, arranged between the object to be examined and an X-ray detector. However, an interference pattern generated by only employing a phase grating may be too small to be detectable with a current X-ray detector, due to a lack of spatial resolution of the X-ray detector.
Thus, a further analyzer grating may be employed arranged between the phase grating and the X-ray detector, subsequently providing an interference pattern, which is large enough to be detectable by current X-ray detectors.
To obtain appropriate image information, a so-called phase stepping is performed. In phase stepping, one of the source grating, the phase grating, and the analyzer grating is displaced laterally with respect to the other grating and the X-ray detector element by a fraction of its grating pitch, e.g., a fourth, sixth, eighth of the grating pitch, e.g. of the phase grating. If the phase stepping is performed using a particular grating, then the phase stepping shall cover a full period of this particular grating.
An alternative to the phase stepping is the so-called slit scanning approach. The object, e.g. a woman's breast, is scanned by a scan arm or gantry movement. The redundancy of the data acquisition by means of an arrangement of a number of parallel detector lines can be exploited to eliminate the need for phase-stepping and the gratings need not be moved with respect to each other. Hence, the phase-acquisition can be implemented in the ordinary scanning motion.
Employing such phase grating, in addition the generation of image data deriving from de-coherent X-ray small angle scattering is enabled, the latter type of imaging also being referred to as “dark-field imaging”.
WO 2012/029005 A1 discloses an apparatus for phase-contrast imaging comprising an X-ray source, a first grating element, a second grating element and an X-ray detector element comprising a plurality of detector pixel elements. An object to be imagined is arrangeable between the X-ray source and the X-ray detector element. The first grating element as well as the second grating element is arrangeable between the X-ray source and the X-ray detector element. The X-ray source, the first grating element, the second grating element and the X-ray detector are operatively coupled for acquisition of a phase-contrast image of the object.
However, such imaging devices can be still improved, in particular in view of the flexibility and adaptability to different patients and operation conditions.